Microwave and millimeter-wave holographic or synthetic aperture imaging techniques have been developed for a wide variety of applications. These applications include concealed weapon detection, radar cross-section (RCS) imaging, ground penetrating radar (GPR), through-wall and inner wall imaging, and non-destructive evaluation. The imaging techniques developed are fully three-dimensional and typically operate by scanning a wide bandwidth radar transceiver over a planar or cylindrical aperture, and using mathematical techniques to focus the data into a three-dimensional image. It is advantageous to use mathematical focusing for these applications because it allows for the use of large apertures and extreme near-field operation, where it would be inconvenient or impossible to use physical focusing elements such as lenses or reflectors. Additionally, scanning the transmitter along with the receiver doubles the resolution relative to fixed transmitters and provides superior illumination quality by using a large diversity of transmitter positions.
Many near-field radar imaging applications require real-time or near-real-time data collection and imaging. Sequentially-switched linear array technology that allows one dimension of a planar or cylindrical aperture to be effectively scanned electronically at high speed has been developed. This is accomplished by sequencing through each element or transmit and receive pairs using microwave or millimeter-wave switching networks connected to the radar transceiver. Mechanical scanning along the dimension orthogonal to the array axis then completes the sampling of a two-dimensional cylindrical or planar aperture. These data can then be reconstructed using, for example, wideband holographic imaging algorithms.
Linear Array Sampling Techniques
The most direct and obvious method of scanning along the array axis is to assume that each antenna is placed uniformly along the axis of the linear array, and can function simultaneously as a transmitter and receiver. This scenario is depicted in FIG. 1. A switching network is used to sequentially select each antenna element and then use it to transmit and receive the wideband microwave/millimeter-wave signal. An antenna spacing of Δ results in an effective spatial sample spacing of Δ. While conceptually simple, this technique has a number of drawbacks. First, the antennae must be spaced very closely, usually on the order of one-half wavelength (λ/2) at the center frequency in order to satisfy the spatial sampling criterion on the aperture. This forces the antennae to be very small, and therefore low-gain, and will frequently cause antenna coupling problems between adjacent or neighboring antennae. An additional problem is that the microwave/millimeter-wave transceiver must be capable of separating the transmit signal from the receive signal. This is possible using directional couplers or circulators, however, these introduce additional losses and do not perfectly isolate the weaker received signal from the much stronger transmitted signal.
A more practical and effective technique separates the system into two linear arrays. One array is dedicated as a transmit array, and the other array is dedicated as a receive array. The transmit and receive arrays have an element spacing of 2Δ and are offset from each other by Δ as shown in FIG. 2. A transmit and receive antenna pair approximately sample the spatial point located half-way between the phase centers of each antenna. This approximation is valid assuming that the transmit-receive antenna spacing is much less than the distance to the scattering target. The array is sequentially sampled by switching on the first transmit antenna and the first receive antenna and collecting the first spatial sample. The second transmit antenna can then be switched on and the second sample collected (with the first receive antenna still switched on). The second receive antenna is then switched on (with the second transmit antenna still on) to collect the third spatial sample. This process is then continued across the array. The switching scheme is described in FIG. 2 in the following way. Dotted lines are used to indicate individual pairs of transmit/receive antennas and x's are used to indicate the locations of the effective spatial sampling positions. This configuration has a number of advantages over the configuration shown in FIG. 1. Antennas are dedicated to be either transmitters or receivers, so no duplexing is required in the transceiver. Antenna spacing of 2Δ results in an effective sample spacing of Δ. This allows for larger antennae to be utilized and reduces the coupling between the antennae. Additionally, this design allows amplifiers to be placed within the switching network to compensate for switching losses, which is not readily accomplished if each element must act as both a transmitter and a receiver. To acquire N spatial samples requires approximately N antennae total (N/2 transmit and N/2 receive). This sampling technique is very effective and has been incorporated into many systems.
Quasi-Monostatic Approximation
Implicit in the development of the linear array sampling technique described by FIG. 2 is the quasi-monostatic approximation. This approximation is that separate transmit and receive antennas placed near each other effectively operate as a single transmit-receive (TR) antenna placed at the midpoint of the line joining the phase centers of each antenna. This approximation is illustrated in FIG. 3, and requires that the range (r) is much greater than the separation of the antennas (D). This approximation introduces a path length or phase error that is not usually significant for the array architecture described in FIG. 2, but will be more significant for the multi-static array architecture developed in this paper. This error is simply due to the round-trip path length difference between the path emanating from the mid-point versus the actual round-trip path. For the general configuration shown in FIG. 4, this path length error isΔl=rT+rR−2re  (1)
This error is dependent on the separation of the antennas and the range and position (angle) to the target. In general, the target position cannot be known prior to imaging, so this error cannot be completely corrected, however, as described later in this section, the error can be largely removed by approximating the direction of wave propagation as being along the antenna axes. For the configuration shown in FIG. 5, the error is
                              Δ          ⁢                                          ⁢          l                =                  2          ⁢                      (                                                                                r                    2                                    +                                                            (                                              Δ                        /                        2                                            )                                        2                                                              -              r                        )                    ⁢          •          ⁢                                          ⁢                      1            8                    ⁢                                    (                              Δ                r                            )                        2                                              (        2        )            which is small for Δ□r. In many cases this error can be made small, and in the array configuration shown in FIG. 2, this error is insignificant because it is identical for each effective sample location and therefore does not affect focusing. In this case, only a very slight range position error might be observed.Interlaced Linear Array Sampling Technique
The sampling technique described above and shown in FIG. 2 is very effective, however, it is very desirable to reduce the number of antenna elements required to sample the width/height of the aperture. The number of samples needed is determined by Nyquist sampling requirements, however, the number of antenna elements can be reduced using special sampling techniques that exploit the quasi-monostatic approximation by using multiple receivers for each transmitter (or vice-versa). One sampling technique uses the same configuration as in FIG. 2, except that the receive array has been thinned by removing every other receive antenna element, as shown in FIG. 6. Each receiver is now used in conjunction with four nearest transmit antenna elements, as shown in the FIG. 6 (dotted lines indicate antenna pairings, and short lines indicate effective sample locations). This results in the same effective sample spacing and density as in the conventional separate transmit and receive array sampling technique described in FIG. 2, however the number of receive elements has been reduced by approximately one-half Therefore collection of N spatial samples would require approximately 3N/4 physical antenna elements. Note that this technique would work equally well with the transmit array thinned, rather that the receive array. This technique can be extended by further reducing the number of receive elements by again removing half of the receivers. This thinning process could be continued to the extreme case where the only receiver elements remaining are the ones located near the ends of the linear array. This extreme configuration would likely not work very well due to the abrupt jump in illumination from the left side of the array to the right side of the array, and due to violation of the assumption that the separation between the transmit and receive antennas is much less that the distance to the imaging target.
A prior invention, U.S. Pub No. 2007/0075889, further developed this method to establish an interlaced sampling linear array sampling technique. In one embodiment of this publication, a single array of transmit antennas is placed between two receive antenna arrays. In this embodiment, as shown in FIG. 5 of the 2007/0075889 publication, each transmit antenna is used in conjunction with the four nearest receivers, two from the upper array (R1) and two from the lower array (R2). Sampling using the transmit array and the R1 receive array results in the samples indicated by the left column x's and sampling using the transmit array and the R2 receive array results in the samples indicated by the right column of x's.
Note that the left and right effective sample columns are offset vertically from each other by Δ/2, and laterally by an amount equal to the horizontal spacing of R1/R2 from the transmit array (D). Since a linear mechanical scan is assumed to complete the scanning of the rectilinear or cylindrical aperture, these offset effective sample locations can be made to align at slightly offset times during the mechanical scan. For example, the transmit array can be sequenced using the R2 receive array to collect the sample locations shown in the right column, then when the array has moved right a distance D, the transmit array can be sequenced using the R1 receive array to collect the sample locations shown in the left column superimposed over the previously collected samples. Thus, the effective sample spacing is Δ/2. The advantage of this system is that collection of N spatial samples requires only approximately N/2 physical antenna elements. This reduces the number of antennas required by approximately one-half compared with the technique described in FIGS. 1 and 2. An additional advantage is that the physical separation of the antennas is larger for a given effective sample spacing, which will allow greater antenna gain and/or greater isolation between antennas. Clearly, the R1 and R2 receive arrays could also be thinned (as in the FIG. 4 configuration) to further reduce the number of antenna elements required.
The thinned-receiver array and interlaced linear array sampling techniques described above provide a powerful means of reducing the number of antennas needed to uniformly sample a linear axis. This reduction is approximately a factor of 2 for the interlaced technique. While powerful, this technique has two primary limitations. First the reduction in the antennas needed is limited. Additional thinning of the receive arrays can reduce this somewhat, however, the transmit array needs to maintain a spacing of 2Δ for effective sampling of Δ/2, which limits the reduction in the number of antennas. The technique does not provide a systematic way to continue to reduce the number of antennas required to densely sample the array axis. An additional concern in some cases is that the technique uses two effective or virtual columns, and relies on mechanical motion to overlay the two sample columns. This is an additional complexity that may not be possible or desirable for many imaging system designs.
What is needed is an improved apparatus for synthetic imaging of an object.